Nucleating agents have conventionally been used in silver halide photographic materials for various purposes. For example, hydrazines are used as nucleus-forming agents in direct positive internal latent image-forming silver halide emulsions, and as agents for increasing sensitivity and/or gradation in negative-working surface latent image-forming silver halide emulsions.
Among various direct positive photographic processes, a process of exposing previously fogged silver halide grains in the presence of a desensitizing agent and a process of exposing a silver halide emulsion having light-sensitive nuclei primarily within silver halide grains and developing it in the presence of a nucleating agent, are most useful. The present invention relates to the latter. Silver halide emulsions having light-sensitive nuclei primarily within silver halide grains and forming a latent image mainly within the grains are called internal latent image-type silver halide emulsions, and are discriminated from silver halide grains forming a latent image primarily on the surface of silver halide grains.
Processes for obtaining direct positive images by surface-developing internal latent image-type silver halide photographic emulsions in the presence of a nucleating agent and photographic emulsions or light-sensitive materials to be used for such processes are known. Examples of such processes, emulsions and materials can be found in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552 and 3,317,322, British Pat. Nos. 1,011,062, 1,151,363, 1,269,640 and 2,011,391, Japanese Patent Publication Nos. 29405/68 and 38164/74 and Japanese Patent Application (OPI) Nos. 16623/78, 137133/78, 37732/79, 40629/79, 74536/79, 74729/79, 52055/80 and 90940/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
In the above-described direct positive image-forming processes, the nucleating agents have been added to a developer, but it has been more popular to add the nucleating agent to photographic emulsion layers or other proper layers of a light-sensitive material.
As the nucleating agents to be added to silver halide light-sensitive materials, hydrazine compounds are most known and are described in U.S. Pat. Nos. 2,563,785, 2,588,982 and 3,227,552. However, in the case of adding these hydrazine compounds to light-sensitive materials, the compounds must be used in considerably high concentrations (e.g., about 2 g per mol of silver). Furthermore, since the nucleating agents migrate from emulsion layers into a developer during development processing, the concentration of the nucleating agent in the emulsion changes to cause uneven photographic density. In addition, with multilayer color light-sensitive materials, unbalanced nucleating effects result between the emulsion layers, which leads to formation of unbalanced colors. In order to overcome these disadvantages, there have been developed hydrazine-type nucleating agents having a substituent or substituents capable of adsorbing onto the surface of silver halide grains. Typical hydrazine-type nucleating agents having an adsorption-accelerating group include thioureabound acylphenylhydrazine-type compounds such as those described in U.S. Pat. Nos. 4,030,925, 4,031,127, 4,139,387, 4,243,739, 4,245,037, 4,255,511 and 4,276,364 and British Pat. No. 2,012,443. Further, there are those compounds which have a heterocyclic thioamido group as an adsorptive group described in U.S. Pat. No. 4,080,207, phenylacylhydrazine compounds having a heterocyclic group having a mercapto group as an adsorptive group such as those described in British Pat. No. 2,011,397B, sensitizing dyes having a nucleating substituent within the molecular structure such as those described in U.S. Pat. No. 3,718,470, and those compounds which are described in Japanese Patent Application (OPI) Nos. 200230/84, 212828/74 and 212829/84, Research Disclosure, No. 23510 (November, 1953), ibid., No. 15162 (November, 1976, Vol. 151) and ibid., No. 17626 (December, 1978, Vol. 176).
In general, hydrazine-type nucleating agents are particularly effective in discrimination since they provide a large difference between maximum density (Dmax) and minimum density (Dmin), but they have the disadvantage in that they require a high pH for photographic processing (pH&gt;11). As nucleating agents capable of exerting their function even at a low pH (pH&lt;11), heterocyclic quaternary ammonium salts are known. Examples thereof are described in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Pat. No. 1,283,835 and Japanese Patent Application (OPI) Nos. 3426/77 and 69613/77. Propargyl- or butynyl-substituted heterocyclic quaternary ammonium salt compounds described in U.S. Pat. No. 4,115,122 are particularly effective nucleating agents for discrimination when used in direct positive silver halide emulsions. However, silver halide emulsions, particularly color light-sensitive materials, contain sensitizing dyes for the purpose of spectral sensitization. Therefore, competition of adsorption onto silver halide emulsion takes place between the sensitizing dye and the heterocyclic quaternary ammonium-type nucleating agent, and since the quaternary salt-type nucleating agents have weak adsorbing ability, they must be added in large quantities. Particularly with multilayer color light-sensitive materials, however, uneven density and unbalanced color may result. Consequently, the quaternary salt-type nucleating agents still have insufficient properties. For the purpose of solving this problem, examples of quaternary salt-type nucleating agents having an AgX adsorption-accelerating thioamido group are reported in U.S. Pat. No. 4,471,044. This patent reports that the amount of the nucleating agent necessary for obtaining enough Dmax can be reduced and a decrease in Dmax with time at high temperature can be reduced by introduction of the adsorptive group. Actually, however, the above effect has not been achieved to a fully satisfactory level.
It is known that a high contrast negative image having a large gamma (&gt;10) can be obtained by processing a surface latent image type silver halide negative-working emulsion using a processing solution with a high pH (&gt;11) in the presence of a hydrazine-type nucleating agent. Such a process is described in, for example, U.S. Pat. Nos. 2,419,975, 4,224,401, 4,168,977, 4,243,739, 4,272,614 and 4,323,643. U.S. Pat. Nos. 4,385,108 and 4,269,929 describe examples of hydrazine-type nucleating agents having a group capable of accelerating adsorption of the agents onto silver halide grains. It is further known that, when a negative-working emulsion combined with a hydrazine compound is processed at a lower pH (&lt;11), enhanced sensitivity is obtained. It is also known that in silver halide negative-working emulsion systems, quaternary ammonium salt-type compounds exert development-accelerating function. This is described in, for example, U.S. Pat. No. 4,135,931, Japanese Patent Application Nos. 114328/77 and 121321/77, German Pat. No. 2,647,940 and Belgian Pat. No. 721,568.
However, in many cases, these hydrazine-type nucleating agents cause a deterioration of graininess and a change of gradation by infectious development. In addition, conventional quaternary ammonium salt-type compounds may elute into a processing solution. Consequently, the hydrazine-type nucleating agent and the quaternary ammonium salt-type compound have not been achieved to a fully satisfactory level in a surface latent image-type silver halide negative-working emulsion.